Acceptable radiation doses may differ according to the part to be imaged depending on the development risk of cancer and their hereditary impact in tissues exposed to radiation during X-ray imaging.
The effective dose (Sv) indicating the acceptable radiation dose is represented by the following formula.Effective dose=total amount Σ all tissues (tissue weighting factor×equivalent dose)
Here, the tissue weighting factor is a numerical value representing relatively the absorbed dose (Gy) which received in each tissue. Tissue weighting factors in the 2007 recommendations by the International Commission on Radiological Protection (ICRP) include 0.12 for the breasts, bone marrow (red), colon, and lungs; 0.08 for the stomach, gonads; 0.04 for the thyroid gland, esophagus, liver, and bladder; 0.01 for the bone surface, skin, brain, and salivary glands; and 0.12 for the remaining tissues and organs. The effective dose becomes 1.00 when these tissue weighting factors are multiplied by each equivalent dose and summed up. With regard to the tissue weighting factor, tissues with larger factors have a higher development risk of cancer, etc. than tissues with smaller factors even with the same equivalent dose. Furthermore, the equivalent dose is a value calculated by multiplying the absorbed dose and the radiation weighting factor. The radiation weighting factor differs according to the kind of radiation. For example, the radiation weighting factor of X-rays is “1.”
As mentioned above, the acceptable radiation dose is set according to the part being imaged (tissues and organs included).
Based on the said ICRP recommendations, health related administrative agencies (such as the Ministry of Health, Labor and Welfare) and health related organizations (such as medical associations in each country) officially define standard doses in terms of the part being imaged, adults, children, sex, physique, laboratory procedure, etc.
Health care professionals (such as radiologists) at each medical institution register a standard dose in the X-ray CT system in advance. FIG. 4 is a table showing an example of standard doses for the chest that is selected as a part to be imaged. Incident surface doses (mGy) are specified for each part to be imaged (chest) and subject attribute (adults, children, and infants) in the standard doses illustrated in FIG. 4.
X-ray CT system users at each medical institution such as medical technologists (hereinafter, referred to as technologists) input the imaging conditions including the tube voltage, tube current, exposure time, part to be imaged, filming range, and filming technique (such as taking multiple images of the same position) in addition to subject (patient) attributes such as age, sex, and weight (physique) before performing an X-ray imaging.
Technologists may input incorrect imaging conditions (human error). Measures to prevent this from happening have been established.
If the radiation dose based on the imaging conditions input by technologists is determined to be inappropriate for medical radiation exposure, systems using conventional technology will alert of inappropriate imaging conditions.
This technology can prevent excessive exposure to radiation by alerting of inappropriate imaging conditions and allowing technologists to correct the imaging conditions if imaging conditions such as the tube voltage and tube current are input incorrectly due to factors such as lack of knowledge and insufficient attention by the technologists.
Systems using conventional technology will alert of inappropriate radiation doses based on the patient information that is input prior to performing an X-ray imaging because the acceptable radiation dose differs depending on subject attributes such as physique, sex, and age.
FIG. 5 is a flow chart indicating a series of processes from inputting imaging conditions to alerting. As shown in FIG. 5, imaging conditions are input by technologists prior to taking an X-ray image (step S101). Then, the subject's attributes are input by technologists (step S102). After that, the standard dose is retrieved (step S103). Next, the X-ray CT system determines whether the radiation dose is appropriate or not by comparing the radiation dose that is calculated based on imaging conditions input by technologists and the subject's attributes to the standard dose (step S104). Then, the system will alert that the imaging conditions are inappropriate (step S105) if, for example, the radiation dose is an excessive radiation dose that is over the reference dose (step S104: Yes).
Furthermore, as an example of a general method for calculating radiation dose based on the imaging conditions, one method calculates the radiation dose based on the part being imaged as well as the tube voltage, tube current, detector mode, helical pitch, and filming range of the X-ray tubes. There is also a method that calculates the radiation dose based on the tube voltage of the X-ray tubes, blade position of the collimator, exposed area, tube current, and X-ray exposure time (for example, Patent Document 1).